Filters are known to provide attenuation of signals having frequencies outside of a particular frequency range and little attenuation to signals having frequencies within the particular range of interest. As is also known, these filters may be fabricated from ceramic materials having one or more resonators formed therein. A ceramic filter may be constructed to provide a lowpass filter, a bandpass filter, or a highpass filter, for example.
For bandpass filters, the bandpass area is centered at a particular frequency and has a relatively narrow bandpass region, where little attenuation is applied to the signals. For example, the center frequency may be at 750 Megahertz (MHz) with a passband region of less than 2 MHz. While this type of filter may work well in some applications, it may not work well when a wider bandpass region is needed or under special circumstances when other characteristics are required.
Block filters typically use an electroded pattern printed on an outer (top) surface of the ungrounded end of the filter in a combline filter design. These top metallization patterns are typically screen printed on the ceramic block, which can be difficult and time consuming in the manufacturing process. Overall, the method of using a metallized pattern on one end of a combline filter can be both costly and labor intensive.
An alternative design technique involves eliminating the need to top print on the block by introducing chamfers into the block. Many block filters include chamfered resonator through-hole designs to facilitate and simplify the manufacturing process. The top chamfers help define the intercell couplings and likewise define the location of the transmission zero in the filter response. This type of design typically gives a response with a low side zero. To achieve a high side transmission zero response, chamfered throughholes are typically placed in the grounded end (bottom) of the ceramic block filter. Thus, a high zero response ceramic filter would typically have chamfers at both ends of the dielectric block. A double chamfer filter is more difficult to manufacture. This is due primarily to the tooling requirements and precise tolerances required in making double chamfered through-holes at the top and bottom surface of the filter. The use of a double chamfered design, like the top print design, is also difficult to manufacture, costly, and labor intensive.
A bandwidth of a filter can be designed for specific passband requirements. Typically, the wider the passband, the lower the insertion loss, which is an important electrical parameter. However, a wider bandwidth reduces the filter's ability to attenuate unwanted frequencies, typically referred to as the rejection frequencies. The addition of a transmission zero in the transfer function at the frequency of the unwanted signal could effectively improve the performance of a ceramic block filter as detailed below.
It would be considered an improvement in the art to provide an interdigital design which is easy to manufacture, requires fewer processing steps and still achieves a high side transmission zero using a very simple design. An interdigital ceramic filter which can be easily manufactured to manipulate and adjust the frequency response, preferably with a high side zero to attenuate unwanted signals, could improve the performance of a filter and would be considered an improvement in ceramic filters.